This invention relates generally to gas turbine engines, and, more specifically, to a system for detection of an instability such as a stall in a compression system such as a fan or a compressor used in a gas turbine engine.
In a turbofan aircraft gas turbine engine, air is pressurized in a compression system, comprising a fan module, a booster module and a compression module during operation. In large turbo fan engines, the air passing through the fan module is mostly passed into a by-pass stream and used for generating the bulk of the thrust needed for propelling an aircraft in flight. The air channeled through the booster module and compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan, booster and compressor rotors. The fan, booster and compressor modules have a series of rotor stages and stator stages. The fan and booster rotors are typically driven by a low pressure turbine and the compressor rotor is driven by a high pressure turbine. The fan and booster rotors are aerodynamically coupled to the compressor rotor although these normally operate at different mechanical speeds.
Operability in a wide range of operating conditions is a fundamental requirement in the design of compression systems, such as fans, boosters and compressors. Modern developments in advanced aircrafts have required the use of engines buried within the airframe, with air flowing into the engines through inlets that have unique geometries that cause severe distortions in the inlet airflow. Some of these engines may also have a fixed area exhaust nozzle, which limits the operability of these engines. Fundamental in the design of these compression systems is efficiency in compressing the air with sufficient stall margin over the entire flight envelope of operation from takeoff, cruise, and landing. However, compression efficiency and stall margin are normally inversely related with increasing efficiency typically corresponding with a decrease in stall margin. The conflicting requirements of stall margin and efficiency are particularly demanding in high performance jet engines that operate under challenging operating conditions such as severe inlet distortions, fixed area nozzles and increased auxiliary power extractions, while still requiring high a level of stability margin throughout the flight envelope.
Instabilities, such as stalls, are commonly caused by flow breakdowns on the airfoils of the rotor blades and stator vanes of compression systems such as fans, compressors and boosters. In gas turbine engine compression system rotors, there are tip clearances between rotating blade tips and a stationary casing or shroud that surrounds the blade tips. During the engine operation, air leaks from the pressure side of a blade through the tip clearance toward the suction side. These leakage flows may cause vortices to form at the tip region of the blade. A tip vortex can grow and spread in the spanwise and chordwise directions on the rotor blades and stator vanes. Flow separations on the stator and rotor airfoils may occur when there are severe inlet distortions in the air flowing into compression system, or when the engine is throttled, and lead to a compressor stall and cause significant operability problems and performance losses.
Accordingly, it would be desirable to have the ability to measure and control dynamic processes such as flow instabilities in compression systems. It would be desirable to have a detection system that can measure a compression system parameter related to the onset of flow instabilities, such as the dynamic pressure near the blade tips or other locations, and process the measured data to detect the onset of an instability such as a stall in compression systems, such as fans, boosters and compressors. It would be desirable to have a mitigation system to mitigate compression system instabilities based on the detection system output, for certain flight maneuvers at critical points in the flight envelope, allowing the maneuvers to be completed without instabilities such as stalls and surges. It would be desirable to have an instability mitigation system that can control and manage the detection system and the mitigation system.